Water has a strong absorption peak at 2450 MHz (it’s what kitchen microwaves use).
[0] https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_...
I also wonder if anyone actually experimented with microwaving their dinner at 10GHz or if that was just an educated guess. X-band magnetrons for radars are widely available though, so I would not be surprised if that was coming from practical experience...
"effective penetration and heating favors wavelengths in the cm range (1–30 GHz). Higher frequencies cannot penetrate sufficiently, and much lower frequencies require very high fields, and can have poor coupling unless making use of direct contact or inductive heating. In the years following WWII, emphasis was thus placed on frequencies in the low GHz region, where both the historic development of microwave power sources and serendipity converged."
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9393469
In 1952, a paper called Tubes for dielectric heating at 915 megacycles said this, "Dielectric heating can be much faster at microwave frequencies. Because leakage radiation is large enough to interfere with communications, heating is best done at assigned frequencies. The bands at 915 and 2,450 megacycles are most useful."
"The plan which is, recommended by the United States provides for certain discrete frequencies, with appropriate guard bands, for those industrial scientific and medical devices which cannot be shielded effectively. This assures a minimum disruption of vital communication services. The allocation of frequencies for this purpose is justified because of the tremendous investments in equipment and the public demand for the various services performed, together with the harmful interference to radio communications which would result if such devices were scattered indiscriminately throughout the spectrum. The frequencies proposed are:
13660 kHz
27320 kHz
40980 kHz
2450 MHz"
http://handle.itu.int/11.1004/020.1000/4.62.51.en.108, page 628.
GE lobbied for 2.4 instead so they could emit a noisy harmonic from a crystal based 1.2 system so the FCC agreed.
Source: national archives documents I researched
water can get shaken up by a wide band of frequencies